The present invention relates to an apparatus and method for supporting or repairing a welded joint between a thermal sleeve and an elbow of a jet pump in a water recirculation system of a boiling water nuclear reactor (BWR).
In a boiling water nuclear reactor, an annular space is defined between the core shroud and the reactor pressure vessel wall. Jet pumps are located in the annular space for recirculating water in the reactor. Each jet pump typically includes a riser conduit, connected to an elbow conduit in the annular space. The elbow conduit is welded to a thermal sleeve which penetrates the reactor pressure vessel wall and supplies water for recirculation through the reactor via a jet pump. The weld between the thermal sleeve and the elbow conduit is typically a full penetration butt weld. The weld joint lies within the confined space of the annulus. Access to the weld joint is highly restricted. Moreover, the weld joint is subjected to the reactor environment and is subject to inter-granular stress corrosion cracking.
The jet pump assemblies in the reactor vessel can flood the core to about two-thirds the core height in the event of a Loss of Coolant Accident (LOCA). If the integrity of the jet pump recirculation piping becomes compromised, such as due to a piping break or separation of the riser pipe elbow conduit from the inlet nozzle thermal sleeve, the jet pump system may lose its ability to flood the core during a LOCA.
Over time, cracks may occur in the weld joint between the thermal sleeve and the elbow conduit. Propagation of cracks in the weld joint can compromise the integrity of the weld joint between the elbow and thermal sleeve. Some leakage flow through a cracked welded joint may be tolerable because water leakage does not degrade the ability of the jet pumps to flood the core. However, propagation of cracks in the weld joint may lead to separation of the thermal sleeve and elbow conduit to the jet pump. Upon separation of the sleeve and elbow, the jet pump recirculation system may be severely degraded due to reduction in coolant flow.
Replacement or repair of the weld joint between the thermal sleeve and the elbow conduit might be accomplished by breaching the piping system external to the reactor in a drywell area of the containment system. This approach would necessitate off-loading the fuel from the reactor and draining the reactor vessel water level down to a level below the recirculation inlet nozzles. The associated thermal sleeve and piping safe-end could then be replaced to restore structural integrity of the recirculation piping system. Such an approach would be costly. Accordingly, there is a long felt need for a method and device to provide weld replacement or repair the weld joint between the thermal sleeve and the elbow.